In sample and hold circuits, it is generally desirable to provide an electrically operated switch that notionally switches between high impedance (off) and low impedance (on) states.
One switch technology that is suited for use in precision converters is a “transmission gate” arrangement. However, such a configuration is made with real field effect transistors whose drain-source resistance RDSon varies with the input voltage at an input terminal of the transmission gate. Therefore in the context of a sampling circuit comprising a transmission gate in series with a sampling capacitor, the series resistance of the transmission gate varies with input voltage, and this is a source of distortion, degrading the total harmonic distortion performance of the sampling circuit, and of subsequent or associated devices such as analog to digital converters.
The “on” resistance of the FETs making up the transmission gate can be reduced by making the transistors wider. However this increases the values of parasitic capacitances associated with the transistor switches, which themselves are non-linear, resulting in an increase in distortion from a secondary distortion mechanism resulting from an interaction between the non-zero impedance of a signal source driving the sampling circuit (whether the operation of the sampling circuit is a “sample and hold” or “track and hold” style of operation) and this increased non-linear capacitance.